Generally, these 6000 errors are issues with QuickBooks Online's API.
Check to make sure that your inventory items have an Income Account, Expense Account, and Cost of Goods Sold Account all selected.
If you have a non-inventory item, check that an Income and Expense account are both selected:
These error messages generally come with some notes, like the one below with various numbers. You can retry import on all the documents if the notes aren't clear about the error.
Unexpected Internal Error. (-30004)
If you continue to get this error, please contact us for support.